Magnetic refrigeration is being considered as an alternative technique to gas compressor technology for cooling and heating based on engineering and economic considerations that indicate that magnetic regenerator refrigerators, in principle, using currently known and available magnetic materials are more efficient than gas cycle refrigerators and thus can yield savings in the cost of operation and conservation of energy.
In the use of magnetic regenerator refrigeration of liquefaction of hydrogen gas, the conventional magnetic refrigerant employed to-date comprises a GdPd alloy. This magnetic refrigerant is disadvantageous from a cost standpoint in that 50% of the alloy comprises expensive Pd metal. A second disadvantage is that the GdPd alloy is somewhat ductile and thus is difficult to crush into fine particles (or powder) which are necessary for efficient heat transfer during the refrigeration cycle. Moreover, although the GdPd alloy exhibits useful magnetic entropy, there are several other heavy lanthanides (e.g. Tb, Dy, Ho, and Er) that exhibit magnetic entropy values approximately 35% larger than that of Gd and thus theoretically offer improved properties for magnetic refrigeration, provided all of the magnetic entropy is associated with the ferromagnetic ordering process on which magnetic refrigeration is based. Magnetic materials including Tb, Dy, Ho, and Er appear to have been neglected as candidate magnetic refrigerant materials as a result of the belief that an appreciable fraction of the magnetic entropy of these materials is associated with crystalline electric field effects, not ferromagnetic ordering, and thus would be less than the magnetic entropy attributed to ferromagnetic ordering observed in Gd in which there are no crystal field effects. Moreover, materials containing Tb, Dy, Ho, and Er exhibit a disadvantageously lower ferromagnetic ordering temperature than the GdPd alloy.
U.S. Pat. No. 4,829,770 describes an attempt to provide a magnetic refrigerant material exhibiting magneto-thermal properties for a magnetic regenerator refrigerator based on the Ericsson thermodynamic cycle. In particular, the patent describes a complex magnetic refrigerant that must include at least three distinct magnetic aluminide compounds in powdered or sintered admixture or in a multi-layered arrangement. The magnetic compounds are selected from aluminides of Gd, Tb, Dy, Ho and Er. The magnetic refrigerant mixture or multi-layer described in the patent is formulated for use specifically in an Ericsson thermodynamic cycle refrigerator and is disadvantageous as a result of its complexity in the number of magnetic compounds involved and in the form of the material as either an admixture or multi-layer.
An object of the present invention is to provide a magnetic refrigerant that exhibits magneto-thermal properties useful for a magnetic regenerator refrigerator using the Joule-Brayton thermodynamic cycle and that can be processed to useful form without the need for complex admixturing or multi-layering.